1. Field of the Invention
The invention relates to a driving force transmitting apparatus including a friction clutch in which friction sliding between clutch plates is lubricated with a lubricant.
2. Description of the Related Art
A driving force transmitting apparatus has been known which is, for example, mounted in a four-wheel drive vehicle enabling switching between a four-wheel driving state and a two-wheel driving state and which includes a friction clutch in which frictional sliding between clutch plates is lubricated with a lubricant (see, for example, Japanese Patent Application Publication No. 2013-100079 (JP 2013-100079 A).
The driving force transmitting apparatus described in JP 2013-100079 A includes a first rotating member and a second rotating member that can rotate relative to each other on the same rotation axis, a friction clutch arranged between the first rotating member and the second rotating member, and a case having a housing portion that houses the first rotating member and the second rotating member. The first rotating member is shaped like a bottomed cylinder inside which the friction clutch is arranged. The second rotating member is shaped like a shaft with one end thereof housed in the first rotating member.
The friction clutch is a wet multi-disc clutch having a plurality of clutch plates that rotates along with the first rotating member and a plurality of clutch plates that rotates along with the second rotating member. Frictional sliding between the clutch plates is lubricated with a lubricant. The case is provided with a tank portion in which a portion of the lubricant is stored.
The driving force transmitting apparatus described in JP 2013-100079 A is configured such that more of the lubricant is stored in the tank portion of the case during two-wheel driving, i.e., when a relatively small amount of lubricant is needed between the clutch plates, than during four-wheel driving. When the driving state is switched from the two-wheel driving to the four-wheel driving, the lubricant is discharged through an oil outlet port formed at a lower end of the tank portion to lubricate frictional sliding between the clutch plates.
In this configuration, during the four-wheel driving, the lubricant suppresses wear of the clutch plates, and during the two-wheel driving, a drag torque resulting from the viscosity of the lubricant is reduced to decrease rotational resistance of the first and second rotating members, improving fuel economy performance.
In the driving force transmitting apparatus described in JP 2013-100079 A, even when the lubricant is discharged through the oil outlet port formed at the lower end of the tank portion, the clutch plates fail to be sufficiently lubricated unless an amount of lubricant in which a part of the friction clutch is immersed is stored in the housing portion of the case. Thus, after the two-wheel driving state is switched to the four-wheel driving state, a long time is needed until the clutch plates are appropriately lubricated with the lubricant discharged from the tank portion.